Analysis and design of steel storage racks subjected to rocking

Abstract

Selective storage racks are used worldwide as a lightweight structure for both temporary and long term storage of palletised goods. These structures are routinely designed for earthquake actions using the equivalent static method, which relies on the fundamental period of vibration to calculate design loads. Selective racks can uplift and rock in the cross-aisle direction during earthquake loading, however, leading to a period of vibration that is longer than the fundamental mode used in conventional design methods. This thesis investigates the uplift and rocking behaviour of selective storage racks and proposes a method to determine the effective period of vibration to improve the accuracy of the equivalent static method. A series of snapback tests were conducted on a full-scale three-level selective rack in order to compare the free vibration/rocking characteristics of four different baseplate types. The period of vibration was shown to increase for increasing rack sway amplitude during rocking and the unanchored rack period could be predicted using Housner’s rocking block model. The damping ratio of the unanchored structure, which was 0.034, could be increased to 0.051 using ductile baseplates that dissipate energy during uplift. A total of 29 shaking table tests were conducted on a similar rack to compare the seismic performance of the unanchored, ductile and heavy-duty baseplates. The rack with unanchored baseplates failed by overturning and the rack with heavy-duty baseplates failed by anchor pull-out; both at 1.5 times the magnitude of the design level ground motion. The ductile baseplate survived all tests, up to 2.3 times the design level ground motion. A finite element model was developed to simulate the rocking behaviour of the upright frame in the cross-aisle direction. The model was validated against experimental testing and used to conduct a parametric study comparing the equivalent static method and the time history analysis method specified in NZS 1170.5:2004. A total of 15 rack configurations, from five baseplate types and 3, 5 and 7 level racks, were analysed. It is recommended that the effective natural period be computed in the Rayleigh method using nonlinear static analysis with the pallet loads included, where the Rayleigh lateral loads create an overturning moment equal to the restoring moment of the pallet loads. Finally, the effect of short duration axial loads, caused by stomping during rocking, on upright members was investigated. Finite element models of 59 uprights were subjected to nonlinear inelastic static and dynamic analyses to determine the effects of length, bracing pitch, cross-section slenderness and torsional restraint on the residual capacity of the member. A typical cold-formed steel rack upright could sustain a 0:1 s stomping force at least 15% greater than its static ultimate capacity without significant reduction in residual capacity, implying that designing against peak loads computed from the time history analysis method may be somewhat conservative.